Inverted Tooth Chain Sprocket with Frequency Modulated Meshing

ABSTRACT

An inverted tooth chain drive system includes a sprocket supported for rotation about an axis of rotation and including a plurality of teeth defined relative to respective tooth centers. The tooth centers are spaced evenly in a circumferential arrangement about the axis of rotation, and each of the plurality of teeth includes an engaging flank. An inverted tooth chain is engaged with the sprocket and includes a plurality of rows of links each structured for inside flank engagement with the sprocket, with leading inside flanks of each row of links projecting outwardly relative to the trailing outside flanks of a preceding row of links. The leading inside flanks of each row are positioned to make initial meshing contact with the engaging flank of one of the sprocket teeth. At least some of the teeth are standard teeth and other ones of the teeth are flank-relieved teeth. The engaging flanks of the flank-relieved teeth are negatively offset relative to their respective tooth centers as compared to said engaging flanks of said standard teeth relative to their respective tooth centers. The root surfaces leading the flank-relieved teeth are raised relative to the root surfaces leading the standard teeth and are also inclined.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from and benefit of the filing date of:(i) U.S. provisional application Ser. No. 60/604,608 filed Aug. 26, 2004(Aug. 26, 2004); and, (ii) U.S. provisional application Ser. No.60/626,276 filed Nov. 9, 2004; and the disclosures of these provisionalapplications are hereby expressly incorporated by reference into thisspecification.

BACKGROUND

Inverted tooth chains have long been used to transmit power and motionbetween shafts in automotive applications and they are conventionallyconstructed as endless chains with ranks or rows of interleaved insidelink plates each having a pair of toes, and having aligned apertures toreceive pivot pins to join the rows and provide articulation of thechain as it drivingly engages the sprocket teeth either at the insideflanks or at the outside flanks of the link plate teeth at the onset ofmeshing with the driving and driven sprockets. Although both meshingstyles have been used for automotive timing drives, inside flankengagement is more commonly used for these drives. Guide link plates arelocated on opposite sides of alternating rows of inside link plates inorder to position the chain laterally (axially with respect to the axisof rotation) on the sprockets.

FIG. 1 shows a conventional inverted tooth chain drive system 100 withan inverted tooth chain 110 in meshing contact with drive sprocket 150as the sprocket rotates clockwise about its center X (axis of rotation),and another sprocket not shown. The sprocket 150 includes a plurality ofteeth 160 each having an engaging flank 162 and the teeth aresymmetrical about their tooth centers TC and are all substantiallyidentical. The sprocket 150 has a total of N teeth, and the toothcenters TC are spaced A degrees from each other, where A=360/N. Theillustrated tooth flanks 162 have an involute form, but canalternatively comprise a radial arc shape and/or comprise or be definedby a straight-sided profile (flat). The outside diameter OD and rootdiameter RD define the outer and inner radial limits of the toothflanks. As shown in FIG. 1, the chain link plates do not contact eitherthe outside diameter OD or the root surface 165 as defined by rootdiameter RD. The teeth 160 are identical to each other and are evenlyspaced circumferentially from each other, with tooth centers TC locatedevery 360/N degrees, where N is the total number of teeth.

FIG. 2A illustrates first and second rows 130 a,130 b of chain 110. Theconventional inside link plates 130 of each row have toes 138 which areeach defined by inside flanks 135 and outside flanks 136 interconnectedby a tip 137 defined by a radius and/or other surface. In theillustrated embodiment, outside flanks 136 are straight-sided and theinside flanks 135 have a convexly arcuate form and are joined by acrotch 134. In particular, the inside flanks 135 of each link 130 aredefined by a radius R that preferably blends into the tip 137 of therelevant toe 138 and into the crotch 134 at the opposite end. When thechain is pulled straight as shown in FIG. 2A (it's nominal orientationas it moves into engagement with the sprocket 150 from the span duringuse), the inside flanks 135 project outwardly from the adjacentoverlapping outside flanks 136 of preceding link row by a projectionheight λ, thereby permitting the inside flanks 135 of a row 130 a,130 bto make initial meshing contact with an engaging flank 162 of a sprockettooth 160 at the onset of meshing. FIG. 2B is a plan view of the chain110 and shows a standard chain lacing having rows 130 a,130 b,130 c,etc. of interleaved inside links 130, with successive rows pivotallyinterconnected by pivot pins 140 or rocker-type joints (the term “pin”is intended to encompass a simple pin or a rocker joint or any structurethat pivotally joins successive link rows 130 a,130 b,130 c. Otherinside link lacings having stacked inside links 130 across a row arealso commonly used.

Referring again to FIG. 1, the chain 110 approaches the drive sprocket150 substantially along the tangent line TL (at the centers of the chainpins 140) in a taut strand and meshing occurs as the chain inside links130 of rows 130 a,130 b,130 c collide with an engaging flank tooth face162. When the chain 110 moves into the wrap of the sprocket and is fullymeshed with the sprocket 150, the centers of the pins 140 travel alongand define a circular path referred to as the pitch diameter PD.

Referring now to FIG. 3, which is an enlarged view of FIG. 1, link platerows 130 a and 130 b of chain 110 are shown at the instant ofsimultaneous meshing contact with the engaging flank 162 of tooth 160 b,i.e., in a state between initial contact with only the leading insideflanks 135 of link row 130 b and transition to engagement only withtrailing outside flanks 136 of a preceding link row 130 a. Link row 130b is making leading inside flank meshing contact IF with tooth flank 162and link plate row 130 a has just rotated into trailing outside meshingcontact OF to affect this simultaneous meshing contact. As sprocket 150continues its rotation, inside flanks 135 of link plate row 130 b willseparate from contact with the engaging flank 162 of tooth 160 b andwill continue to further separate until the sprocket rotationarticulates link plate row 130 b to its chordal position in the sprocketwrap, which occurs when its trailing outside flanks 136 come intomeshing contact OF with engaging flank 162 of tooth 160 c. It should benoted that the transition from leading inside flank contact of link row130 b to trailing outside flank-to-tooth contact of preceding link row130 a, as described, is not believed to contribute in any significantmeasure to the meshing impact noise levels in that the initial meshingand driving engagement of the chain links with the sprocket teeth 160occur at the inside flanks 135 at the onset of meshing, and it is thisinitial chain-sprocket meshing impact that is believed to be the majornoise source. The meshing cycle for a link row starts with initialmeshing contact IC and ends when the link row articulates to, and isseated at, its chordal position in the sprocket wrap, having onlytrailing outside flank contact OF.

It is important to note that initial contact IC between the chain 110and sprocket 150 is always inside flank meshing contact IF. Inside flankcontact IF (see FIG. 3) continues even after the initial contact IC, asshown in FIG. 4, since initial contact by definition occurs at theinstant when the leading inside flanks 135 of a chain row first makeinside flank meshing contact IF with a sprocket tooth 160, and theinside flanks 135 of the chain link row remain substantially in contactwith the engaging flank 162 of a sprocket tooth 160 until the meshingtransition to the outside flank meshing OF occurs, following which theinside flanks 135 will separate from contact with the tooth face 162.

Referring again to FIG. 4, the drive sprocket 150 has continued torotate in a clockwise direction, relative to the position shown in FIG.3, until link plate row 130 c is at the onset of initial meshing contactIC with sprocket tooth 160 c. The angle θ is shown to be the anglebetween a base reference line VL originating at the sprocket center(axis of rotation X) and passing through the sprocket teeth at the 12o'clock (i.e., top-dead-center) position, and another reference line CLoriginating at the sprocket center X and passing through the initialmeshing contact point IC at tooth 160 c, and this is the angle at whichinitial meshing contact IC will occur between the leading inside flanks135 of a chain row 130 a,130 b, etc. and any tooth 160 in thesymmetrical drive sprocket 150, i.e., at the instant of initial contactbetween a row of chain link plates 130 and a sprocket tooth 160, theangle θ will always be defined as the angle between the base referenceline VL and the second reference line CL extending between the sprocketcenter and the initial meshing contact point IC.

Chain-sprocket impact at the onset of meshing is the dominant noisesource in chain drive systems and it occurs as the chain links leave thespan and collide with a sprocket tooth at engagement. Transversevibration in the “free” or unsupported span as the chain approaches thesprocket along the tangent line TL will add to the severity of themeshing impact. The resultant impact noise is repeated with a frequencygenerally equal to that of the frequency of the chain meshing with thesprocket. Many attempts to reduce the noise associated with invertedtooth chain drives have been related to the chain-sprocket meshingphenomenon. It is well known in the art that an inverted tooth chainhaving inside flank meshing will generally provide a smoothchain-sprocket engagement. The noise generation associated withchain-sprocket meshing impact, however, still occurs for inside flankmeshing contact and it is an object of the present invention to reducethese noise levels.

SUMMARY

In accordance with the present development, an inverted tooth chaindrive system includes a sprocket supported for rotation about an axis ofrotation and comprising a plurality of teeth defined relative torespective tooth centers. The tooth centers are spaced evenly in acircumferential arrangement about the axis of rotation, and each of theplurality of teeth includes an engaging flank. An inverted tooth chainis engaged with the sprocket and includes a plurality of rows of linkseach structured for inside flank engagement with the sprocket, withleading inside flanks of each row of links projecting outwardly relativeto the trailing outside flanks of a preceding row of links. The leadinginside flanks of each row are positioned to make initial meshing contactwith the engaging flank of one of the sprocket teeth. At least some ofthe teeth are standard teeth and other ones of the teeth areflank-relieved teeth. The engaging flanks of the flank-relieved teethare negatively offset relative to their respective tooth centers ascompared to said engaging flanks of said standard teeth relative totheir respective tooth centers.

In accordance with another aspect of the present development, a sprocketis adapted to mesh with an inside flank engagement inverted tooth chain.The sprocket includes a plurality of teeth defined relative torespective tooth centers, wherein the tooth centers are spaced evenly ina circumferential arrangement about said axis of rotation, and whereineach of said plurality of teeth includes an engaging flank. At leastsome of the teeth are standard teeth and others of the teeth areflank-relieved teeth, with the engaging flanks of the flank-relievedteeth negatively offset relative to their respective tooth centers ascompared to the engaging flanks of the standard teeth relative to theirrespective tooth centers.

In accordance with another aspect of the present development, a methodof meshing an inverted tooth chain with a sprocket includes rotating asprocket while teeth of the sprocket are engaged with an inverted toothchain so that leading inside flanks of each row of links of the invertedtooth chain make initial contact with an engaging flank of a sprockettooth and, after making initial contact, fully mesh with the sprocket,wherein the sprocket comprises: (i) a plurality of standard teeth havingstandard engaging flanks; and, (ii) a plurality of flank-relieved teethhaving flank-relieved engaging flanks that are negatively offsetrelative to the standard engaging flanks. The step of rotating thesprocket includes rotating the sprocket a first angular distance so thata first row of links of the chain fully meshes with a first standardtooth. The sprocket is rotated a second angular distance so that asecond row of links of the chain makes said initial contact with asecond standard tooth that is preceded by the first standard tooth. Thesecond angular distance measured from an instant when the first row oflinks of the chain first becomes fully meshed with the first standardtooth. The sprocket is rotated a third angular distance so that a thirdrow of links of the chain makes initial contact with a flank-relievedtooth that is immediately preceded by the second standard tooth, whereinthe third angular distance is measured from an instant when the secondrow of links of the chain first becomes fully meshed with the secondstandard tooth,

wherein the third angular distance exceeds the second angular distance.

BRIEF DESCRIPTION OF DRAWINGS

The invention comprises various components and arrangements ofcomponents, preferred embodiments of which are illustrated in theaccompanying drawings wherein:

FIG. 1 is a partial front elevational view of a chain drive systemcomprising a conventional inverted tooth chain (with some of the guidelinks removed for clarity) meshing with a conventional inverted toothsprocket with inside flank meshing;

FIG. 2A is a greatly enlarged illustration of first and second rows ofinside link plates of the inverted tooth chain shown in FIG. 1;

FIG. 2B is a top plan view of the inverted tooth chain of FIG. 1;

FIG. 3 is an enlarged view of the chain drive system of FIG. 1;

FIG. 4 corresponds to FIG. 3 with the sprocket rotated to a newposition;

FIG. 5A is a partial front elevational view of a chain drive systemcomprising a conventional inverted tooth chain (with some guide linksremoved for clarity) meshing with a sprocket formed in accordance withthe present invention;

FIG. 5B is an enlarged view of the chain drive system of FIG. 5A;

FIG. 5C corresponds to FIG. 5B with the sprocket rotated to a newposition;

FIG. 5D is a partial enlarged view of the sprocket of FIG. 5A withoutshowing the chain;

FIG. 6 is a greatly enlarged illustration of a flank-relieved tooth ofthe sprocket shown in FIG. 5A, with the conventional tooth profile shownas an overlay using a phantom line;

FIGS. 7A, 7B, 7C, 7D, and 7E are greatly enlarged views of the chaindrive system of FIG. 5A and illustrate frequency modulated meshing ofthe chain with the sprocket according to the present invention;

FIG. 7F is a greatly enlarged view of the chain drive system of FIG. 5Aand illustrates a chain having a link pitch at the as-manufactured highlimit meshing with a sprocket according to the present invention;

FIG. 8A is a front elevational view of the sprocket of FIG. 5A in itsentirety, with the teeth numbered for reference herein;

FIG. 8B graphically illustrates the pattern of conventional andflank-relieved teeth and initial meshing contact angles for the sprocketof FIG. 8A;

FIG. 8C graphically illustrates the pattern of conventional andflank-relieved teeth and initial meshing contact angles for a chain witha slightly elongated pitch length meshing with the sprocket of FIG. 8A;

FIG. 9A is a front elevational view of a complete sprocket formed inaccordance with another embodiment, with the teeth numbered forreference herein;

FIG. 9B is a greatly enlarged illustration of a second embodiment of aflank-relieved tooth for the sprocket of FIG. 9A, with a conventionaltooth profile overlay shown using a phantom line;

FIGS. 10A, 10B, 10C, and 10D are greatly enlarged views of a chain drivesystem having the sprocket of FIG. 9A, including multiple differentflank-relieved teeth and conventional symmetrically formed teeth, andillustrates frequency modulated meshing of the chain with the sprocketaccording to the present invention;

FIG. 11 graphically illustrates the pattern of conventional andflank-relieved teeth and initial meshing contact angles for a chain witha slightly elongated pitch length meshing with the sprocket of FIG. 9A.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 5A, and more clearly in FIG. 5B, inverted tooth chaindrive system 200 is comprised of inverted tooth chain 210 in meshingcontact with a drive sprocket 250 as the sprocket rotates clockwise, andanother sprocket not shown. The illustrated chain 210 is identical tothe chain 110 and, thus, like components of the chain 210 relative tothe chain 110 are identified with reference characters that are 100greater than those used in connection with the chain 110. It is notintended that a sprocket formed in accordance with the present inventionbe limited for use with the illustrated inverted tooth chain or anyother particular style of inverted tooth chain. The inverted tooth chainsprocket 250 is formed in accordance with the present invention withtooth flanks having an involute form but the flanks may include a radialform instead of an involute form and/or can comprise or be defined byone or more flats without departing from the overall scope and intent ofthe present invention.

Referring specifically to FIG. 5B, which is an enlarged portion of FIG.5A, link plate rows 230 a and 230 b are shown to have simultaneousmeshing contact with the engaging flank 262 of tooth 260 b. Leadinginside flanks 235 of row 230 b are making inside flank meshing contactIF and link plate row 230 a has just rotated sufficiently so that itstrailing outside flanks 236 move into outside meshing contact OF toeffect this simultaneous contact. This simultaneous contact is definedas “transition contact” and, with the next increment of rotation ofsprocket 250, inside flanks 235 of link plate row 230 b will start toseparate from the engaging flank 262 of tooth 260 b, and will continueto further separate from the tooth until the sprocket rotationarticulates link plate row 230 b to its chordal position in the sprocketwrap, which occurs when its trailing outside flanks 236 come intomeshing contact OF with engaging flank 262 of tooth 260 c. The terms“leading” and “trailing” as used herein are used with reference to thedirection of rotation of the sprocket 250 and movement of the chain 210,so that the phrase “leading inside flanks 235” refers to the insideflanks 235 of a chain link row 230 that are first to encounter thesprocket 250.

As illustrated in FIG. 5C, the sprocket 250 is rotated in a clockwisedirection relative to the position shown in FIG. 5B until the leadinginside flanks 235 of link row 230 c are at the onset of initial meshingcontact IC with engaging flank 262 of tooth 260 c. Teeth 260, i.e., 260a,260 b,260 c, etc. are standard sprocket teeth such as the teeth 160described above in connection with FIGS. 1-4 and, thus, serve to definethe initial contact angle θ for sprocket 250, and this angle is equal tothe angle previously defined for conventional sprocket 150 having a fullcomplement of standard sprocket teeth 160 (of course, the angle θ willvary from application to application depending upon the sprocket chordalpitch, the number of sprocket teeth, and/or other factors). The angle θis defined relative to the 12 o'clock base reference line VL as definedabove. Referring still to FIG. 5C, link plate row 230 d will follow linkplate row 230 c as the next link row to mesh with sprocket 250 and itwill mesh with tooth 280, which is formed in accordance with the presentinvention to have an engaging flank profile 282 that is dimensionedand/or shaped differently as compared to teeth 260.

As shown in FIG. 5D, and more clearly in FIG. 6, tooth 280 has anadjacent leading root surface 285 (leading in terms of direction ofrotation of sprocket) that is positioned radially outward of the rootsurface 265 of the standard tooth form, and an engaging flank 282 isnegatively offset or “relieved” relative to the engaging flank 262 ofteeth 260 in terms of the sprocket rotation direction as shown tobeneficially provide flank relief FR1. In FIG. 6, engaging flank 262 ofconventional tooth 260 is shown overlaid with flank-relieved tooth 280.This engaging flank relief FR1 is preferably in the range of 0.03-0.10mm. It is important to note that the tooth center TC for tooth 280 isdefined to be coincident with that of a non-flank relieved standardprofile tooth 260 when the profiles are overlaid as shown. Withcontinuing reference to FIG. 5D, the root surfaces 265 of conventionalteeth 260 are tangent to the inscribed circle defined by root diameterRD. The root surface 285 forward of the engaging flank 282 of tooth 280is shown to be positioned radially outward of the conventional rootsurface 265 and this is done in order to place a chain link row 230 thatis fully meshed with a tooth 280 in its proper radial position in thesprocket wrap (see FIG. 7B, in which link row 230 c has just articulatedto its chordal position). Referring again to FIG. 6, this adjacentleading root surface 285 is clearly shown to incline radially inwardtoward the axis of rotation extending forward of tooth 280 away from therelieved engaging flank 282 toward a preceding tooth 260 and the reasonfor the incline will be explained below. The root surfaces 285 aresometimes referred to herein as “raised root surfaces.”

It should be noted that the flank-relieved teeth 280 are not definedsymmetrically about the tooth centers TC, owing to the negative offsetof the engaging flank 282. As such, for a flank-relieved tooth 280 (or astandard tooth 260), the tooth center TC can also/alternatively bereferred to as the tooth origin, i.e., the terms “tooth center” and“tooth origin” both describe the circumferential tooth location located360/N degrees from the corresponding location on both preceding andsucceeding teeth, where N equals the total number of teeth 260,280.

Referring now to FIG. 7A, inside link plate row 230 d is shown at theinstant of initial meshing contact IC with engaging flank 282 offlank-relieved tooth 280, and sprocket 250 had to rotate an additionalamount, angle Δ₂₆₀₋₂₈₀, for initial meshing contact IC to occur betweenleading inside link flanks 235 of link row 230 d and engaging flank 282of tooth 280 owing to the flank relief FR1 of tooth 280. Thiseffectively and advantageously delays the initial meshing impact of linkrow 230 d with tooth 280, thereby serving to modulate the meshingfrequency of the drive sprocket 250. This will be apparent to those ofordinary skill in the art upon reviewing FIG. 7A where a standard toothform 260 is overlaid in phantom lines on the tooth 280 at an angularposition where initial contact IC with link row 230 d would have beenmade with the standard tooth engaging flank 262 but for the relievedengaging flank 282 as compared to the flank. The inside flanks 235 ofchain row 230 d would have made initial contact with the standard tooth260 at the angular position shown by the phantom line CL₂₆₀ but, becauseof the presence of the flank-relieved tooth 280, the sprocket had torotate through the additional angle Δ₂₆₀₋₂₈₀ in order to make initialcontact with chain 210 as indicated by the line CL₂₈₀.

Referring still to FIG. 7A, it should be noted that link row 230 b is inits seated chordal (fully meshed) position and, thus, pin center 241connecting link rows 230 b and 230 c is in the sprocket wrap on thepitch diameter PD, with pin center 242 joining rows 230 c,230 d shown tobe close to, but not yet in the wrap at the initial meshing contact ICof link row 230 d.

As illustrated by FIG. 7B, where sprocket 250 is shown rotated forwardrelative to FIG. 7A, the leading inside flanks 235 of link row 230 dmaintain tooth contact IF as the sprocket continues to rotate until thetransition point for this root relieved tooth 280, which is shown tooccur when toe tips 237 of trailing toes 238 of row 230 c contact rootsurface 285 at root contact location RC, when row 230 c is fully meshed,seated in its chordal position. When trailing toes 238 of link row 230 ccontact root surface 285, row 230 c is then in its chordal position inthe sprocket wrap and it should be noted that there is no outside flankmeshing contact of its trailing outside flanks 236 with engaging flanks282 of tooth 280, due to the flank relief FR1 (see FIG. 7C). Rootsurface 285 beneficially maintains link plate row 230 c in hard contactwith sprocket 250 at its chordal position, and pin center 242 is thusheld on the pitch diameter PD. In other words, contact between chain 210and root surface 285 places link row 230 c in substantially the sameposition that it would occupy if were in full meshing contact in itschordal position with a standard tooth form 260 and prevents freefloating of the chain 210.

FIG. 7C shows the sprocket 250 positioned identically to FIG. 7B, butuses progressive phantom lines 230 d-1,230 d-2 to illustrate the fullrotation of the link row 230 d as it moves from its initial contactlocation into its fully meshed chordal position upon rotation of thesprocket (rotation of sprocket is not shown). It can be seen that as thelink row 230 d rotates about pin center 242 upon sprocket rotation, theinside flanks 235 of the leading toes 238 (i.e., the leading insideflanks 235) of link row 230 d progressively separate from the sprockettooth flank 282 and tips 237 of the leading toes 238 of all inside links230 in row 230 d move forward on or just above declining root surface285 without any cam-action against surface 285 as would cause pin center242 to move radially outward as indicated in the phantom lines. Ofcourse, the tips 237 of leading toes 238 of row 230 d must not includeany lobes/projections as would cause lifting of pin center 242 away fromsurface 285. The “downhill” incline of root surface 285 forward of atooth 280, therefore, ensures that the moving toe tips 237 of row 230 ddo not act in a cam-like fashion on root surface 285 during thisaforementioned rotation of the leading toes 238 as would cause pincenter 242 to be lifted above the pitch diameter PD as trailing pincenter 243 moves onto the pitch diameter PD.

Referring now to FIG. 7D, where sprocket 250 is rotated forward anotherincrement relative to FIG. 7B, link row 230 e is shown at the onset ofinitial meshing contact IC with a standard tooth 260 e followingpreceding link rows 230 c and 230 d. Link row 230 c has already fullymeshed with flank-relieved tooth 280 and is held at its chordal positionin the sprocket wrap due to contact between its trailing toes 238 androot surface 285 as just described. FIG. 7E shows another incrementalrotation of sprocket 250 to a point of transition (simultaneous) contactbetween link rows 230 d,230 e and also illustrates movement of leadingtoes 238 of row 230 d relative to root surface 285 with no radialdisplacement of pin center 242 due to incline of root surface 285. It isimportant to note that for the meshing geometry illustrated in FIG. 7D,chain link pitch P_(C) is equal to theoretical pitch, or 7.7 mm in thisinstance, resulting in link row 230 c being substantially in the sameradial and rotational position as it would be if it were meshed with astandard tooth 260. Accordingly, the initial contact angle θ for thelink row 230 e with standard tooth 260 e is substantially the same aspreviously defined for any meshing with a standard tooth 260, i.e., thepresence of flank-relieved tooth 280 preceding tooth 260 d does notchange the initial contact angle θ due to the root surface 285 and alsodue to the fact that, in the illustrated example, the chain link pitchP_(C) is equal to a theoretical pitch.

A new as-manufactured chain will have a link pitch length P_(C) thatwill fall within specified manufacturing tolerance limits, and typicallythe low limit will be at, or very close to, minimum theoretical pitchP_(c), and the high limit will approximately equal 1.0009 timestheoretical pitch. It is well known in the chain manufacturing art, andis important to note, that link plates are typically batch processedand, accordingly, all link plates used to make each chain will havesubstantially the same link pitch P_(C), and this link pitch will fallwithin the manufacturing tolerance limits as defined. Referring now toFIG. 7F, in real-world conditions, the system 200 will comprise a chain210′ that is identical to the chain 210 except that it has a link pitchP_(C)′ that is equal to a high limit as-manufactured chain, or 0.09%pitch elongation over minimum theoretical pitch. Link row 230 f is shownin full meshing contact with sprocket 250 via contact between itstrailing toe tips 237 and root 285 of flank-relieved tooth 280 atlocation RC. Although the link row 230 f is in meshing contact withsprocket 250, the trailing outside flanks 236 thereof do not contactengaging flank 282 due to flank relief FR1 (FIG. 6), and, accordingly,this link row 230 f is free to move closer to the engaging flank 282 asa function of the elongated link pitch P_(C)′. The next link row, 230 g,has already had its initial meshing contact IC with flank-relieved tooth280 and has separated from its inside flank IF contact. The next linkrow, 230 h, is at the onset of initial meshing contact IC, and thismeshing contact with standard tooth 260 is advanced by the angleΔ₂₈₀₋₂₆₀, and this meshing modulation in the advanced direction (withrespect to θ) is a result of the elongated pitch P_(C)′ in combinationwith the flank-relieved tooth 280 as described above.

Referring still to FIG. 7F, it is important to note that the initialmeshing contact IC occurs earlier when a flank-relieved tooth 280precedes a standard tooth 260 and the chain link pitch is greater thantheoretical pitch, thereby advancing the meshing by an amount equal toangle Δ₂₈₀₋₂₆₀. The angle Δ₂₈₀₋₂₆₀ is defined as the angle between theradial lines CL_(260′) and CL₂₆₀ where the line CL₂₆₀ passes through thecenter (axis of rotation X) of sprocket 250 and the initial contactlocation IC between tooth 260 and chain 210′, and where the lineCL_(260′) passes through the center of the sprocket and the point wherethe tooth 260 would make initial contact with chain 210′ if not precededby the flank-relieved tooth 280. Those of ordinary skill in the art willrecognize that this advancement of the initial meshing contact (increasein the angle Δ₂₈₀₋₂₆₀) when a flank relieved tooth 280 precedes astandard tooth 260 (assuming a chain pitch P_(C)′ longer than P_(C)) isamplified even further when two or more successive flank relieved teeth280 precede a standard tooth 260, at least to a point where furtherincreases in Δ₂₈₀₋₂₆₀ are not possible due to contact between the chain210′ and the relieved flank 282 of a preceding tooth 280.

Referring now to FIG. 8A, conventional teeth 260 and flank-relievedteeth 280 are arrayed around the sprocket 250 in a random or a specificpattern with the tooth centers/origins TC spaced evenly at 360/N degrees(where N is the total number of teeth) in order to optimize the meshingmodulation for a particular engine chain drive, but other patterns andtooth combinations are within the scope of this invention includingpositioning two or more flank relieved teeth 280 next to each otherand/or two or more conventional teeth 260 next to each other.

The meshing modulation for the sprocket 250 and a chain 210 isillustrated in FIG. 8B for a chain having a link pitch equal to atheoretical, or 7.7 mm pitch in this instance, and the 30-tooth drivesprocket 250. It can be seen that for the sprocket 250 shown in FIG. 8A,θ=13° as between successive standard teeth 260 (for “normal” initialcontact frequency) and θ=11.75° as between a standard tooth 260 followedby a flank-relieved tooth 280 in the direction on rotation (for delayedinitial contact). For the theoretical chain pitch P_(c), θ=13° asbetween a flank-relieved tooth 280 followed by a standard tooth 260(normal initial contact frequency) but, as just noted, under real-worldconditions, an elongated chain pitch P_(C)′ in a chain 210′ will resultin θ>13° (for advanced initial contact) for additional modulation of theinitial contact meshing frequency when at least one flank-relieved tooth280 precedes a standard tooth 260. This can be seen with reference toFIG. 8C which illustrates meshing geometry utilizing a chain having0.09% pitch elongation (a high limit as-manufactured chain), and meshingmodulation is enhanced to include advancing the initial meshing contactsIC with a chain having elongated link pitch P_(C)′. In particular, FIG.8C shows that for the same 30 tooth sprocket of FIG. 8B, θ=13° asbetween successive standard teeth 260, θ=11.75° as between a standardtooth 260 followed by a flank-relieved tooth 280 (for delayed initialcontact), and θ=13.13° when a standard tooth 260 follows aflank-relieved tooth 280 (for advanced initial contact).

FIG. 9A shows a sprocket 350 formed in accordance with the presentdevelopment that is identical to the sprocket 250 except as otherwiseshown and/or described and, as such, like features of sprocket 350relative to sprocket 250 are identified with reference numbers that are100 greater than those used herein to describe the sprocket 250. Thesprocket 350 comprises standard teeth 360, one or more firstflank-relieved teeth 380 and also one or more second flank-relievedteeth 390. Each of the second flank relieved teeth comprises an engagingflank 392 defined with flank relief that is greater than that forengaging flank 382 of tooth 380. Tooth 390 has an engaging flank 392that is offset relative to an engaging flank 262 of a standard tooth 260to provide flank relief FR2 as shown in FIG. 9B. This flank relief FR2is preferably in the range of 0.05-0.15 mm. Root surface 395 offlank-relieved tooth 390 is preferably identical to root surface 285 offlank-relieved tooth 280 and acts in the same fashion to maintain hardcontact between a chain 210,210′ and sprocket 350 forward of aflank-relieved tooth 390 and is inclined radially inwardly movingforward from tooth 390 to prevent camming action and lifting of the pin240 located in front of tooth 390 when the leading toes 238 of atrailing link row move relative thereto as described above.

FIGS. 10A through 10D progressively show engagement of a chain 210′ withsprocket 350. Referring first to FIG. 10A, inside link plate row 230 cis shown at the instant of initial meshing contact IC with engagingflank 392 of flank-relieved tooth 390 of the sprocket 350. The toothpreceding tooth 390 is a standard tooth 360. As shown, sprocket 350 mustrotate further through an added angle, Δ₃₆₀₋₃₉₀ before initial meshingcontact IC occurs (reduction in the angle θ) as compared to the initialcontact angle θ defined when successive conventional teeth 360 are usedas indicated by the trailing tooth 360 shown in phantom lines. Thepresence of a trailing flank-relieved tooth 390 delays the inside linkplate row 230 c meshing impact with tooth 390, thereby serving tofurther modulate the meshing frequency beyond that of tooth 380.

In FIG. 10B the sprocket 350 has been rotated forward to a position oftransition contact for a flank-relieved tooth, where the trailing toes238 of link row 230 b contact root surface 395 at RC, and leading insideflanks 235 of link row 230 c are still in contact at IF with flank 392of tooth 390. Because the flank 392 is relieved as described, thetrailing outside flanks 236 of link row 230 b will not make transitioncontact with the flank 392. Instead, the transition occurs as shown whentrailing toes 238 of row 230 b contact root surface 395 at location RCbefore inside flanks 235 of link row 230 c move out of contact withsprocket tooth flank 392.

FIG. 10C shows the sprocket 350 rotated forward another increment so itcan be seen that with link row 230 b fully meshed with the sprocket,trailing outside flanks 236 thereof are spaced from relieved flank 392and trailing toes 238 thereof maintain contact with root surface 395 atRC to maintain pin center 241 on the pitch diameter PD. Again, it isnoted that inclined root surface 395 ensures that leading toes 238 offollowing link row 230 c do not cam against root surface 395 as wouldlift pin center 241.

As shown in FIG. 10D, the meshing initial contact IC occurs earlier(increase in the angle θ), thereby advancing the meshing by an amount,angle Δ₃₉₀₋₃₆₀ when a flank-relieved tooth 390 precedes a standard tooth360 (or a tooth 380) at the onset of meshing for a chain 210′ having alink pitch P_(C)′ equal to a high limit as-manufactured chain, or 0.09%pitch elongation over theoretical pitch. The angle Δ₃₉₀₋₃₆₀ is definedas the angle between the radial lines CL_(360′) and CL₃₆₀ where the lineCL₃₆₀ passes through the center of sprocket 350 and the point of initialcontact between tooth 360 and chain 310′, and where the line CL_(360′)passes through the center of the sprocket and the point where initialcontact with tooth 360 would be made with chain 310′ if not preceded bythe flank-relieved tooth 390. Those of ordinary skill in the art willrecognize that this advancement of the initial meshing contact (increasein the angle θ by an amount=Δ₃₉₀₋₃₆₀) when a flank relieved tooth 390precedes a standard tooth 260 (assuming a chain pitch P_(C)′ longer thanP_(C)) is amplified further when two or more successive flank relievedteeth 280,390 precede a standard tooth 260, at least to a point wherefurther increases in Δ₃₉₀₋₃₆₀ are not possible due to contact betweenthe chain 210′ and the relieved flank 282,392 of a preceding tooth280,390.

Teeth 360,380 and 390 can be arrayed around the sprocket 350 in a randomor specific pattern in order to optimize the meshing modulation. Onesuch pattern is illustrated graphically in FIG. 11 for a 30-toothsprocket, but other patterns and tooth combinations are within the scopeof this invention including positioning two flank relieved teeth next toeach other. It is also within the scope of this invention to modify thesprocket tooth profile to include added pitch-mismatch in order toincrease the meshing modulation angle with the meshing impact ICoccurring sooner (advanced) when meshing with a standard tooth form ispreceded by a flank-relieved tooth form. FIG. 11 shows that, for a chain210′ having a pitch P_(C)′ longer than the theoretical pitch P_(C) theangle θ=13° for successive standard teeth 360, θ=11.10° for a standardtooth 360 followed by a flank-relieved tooth 390 (delayed initialcontact), θ=11.75° for a standard tooth 360 followed by a flank-relievedtooth 380 (delayed initial contact), θ=13.13° for a singleflank-relieved tooth 380,390 followed by a standard tooth 360 (advancedinitial contact).

A sprocket formed in accordance with the present development can be cutfrom steel stock or can be defined using a powder metallurgy (“PM”)process, many of which are well known in the art. Such PM processesutilize the steps of mixing of the alloy components, compacting thepowder metal in a die, and sintering the compacted part. Because of theenvironment in which they operate and the stresses they are subjectedto, sprockets for use in automotive applications have traditionally alsorequired a heat treating or other hardening step subsequent tosintering, i.e., sprockets for such automotive applications must havephysical properties (hardness, tensile and impact strength) higher thanrequired by sprockets for less demanding applications and, thus, aseparate heat treating operation is normally required for durability andwear considerations.

Conventional heat treating of PM components commonly involves heatingthe entire part to a temperature of approximately 1550° F. thenquenching the parts rapidly in either oil, polymer/water mixture ornitrogen gas. Alternatively, some PM sprockets are induction heattreated. This involves heating the sprocket teeth to a depth ofapproximately 4 millimeters below the root of the tooth. The sprocketsare then quenched rapidly in either oil, a polymer/water mixture, orair. Unfortunately, such hardening processes can negatively affect thetolerances achievable for such sprocket profiles due to dimensionalchange and distortion. The amount of distortion is related to the amountof heat used in such processes. In an attempt to minimize the amount ofheat used, the induction heat treating process uses multiple inductionfrequencies to concentrate the approximate 1550° F. heat at the toothsurfaces, thereby reducing the total heat in the part. Although partdistortion and dimensional change are minimized in this manner, they arestill experienced.

Because the design features of the above-described random tooth sprocketthat generate superior NVH properties are very small and the tolerancesquite tight, it is important that the entire process is engineered forhigher precision than conventional PM sprockets are capable using asintering step and subsequent heat hardening. By using asinter-hardening process as opposed to the above-described hardeningprocesses, subsequent induction hardening or other hardening of theteeth after sintering is not required, which beneficially permits one tomaintain better control for the required tooth profile featuretolerances, particularly with flank relief profiles. One suitablesinter-hardening process for use in the production of the abovedescribed sprockets may utilize an alloy having a specific compositionas well as a sintering process using a certain atmosphere, temperature,and subsequent rapid cooling rate. A suitable alloy for use in thepresent sinter-hardening process includes the following, whereinpercentages are weight percent: copper 1.5-2.2%; nickel 1.2%-1.6%;molybdenum 1.0-1.5%; graphite 0.7-1.0%; manganese 0.3-0.6%; lubricant0.5%-0.8%; and the balance iron. The nickel, molybdenum, and manganeseare prealloyed and introduced into the iron melt. The resulting alloy isthen atomized to form an alloy powder. The remaining ingredients arethen admixed in to form the powder blend to be used in the process. Inaddition, it is known in the PM industry that varyingconcentrations/non-homogeneous distribution of fine particle alloyingelements such as nickel and graphite have an detrimental affect ondimensional change and variability. To reduce the loss of small nickeland graphite admixed alloying particles in the powder transfer systemand to prevent segregation, a binder-treatment may be applied to thepowder mixture. The binder treatment adheres small alloying particles tolarger particles preventing loss due to dusting and concentrations offine particles due to segregation.

The powder blend is then compacted in a tool or die using knowncompacting processes. In one embodiment, the temperature of the powderand/or tooling is between 250-280° F. After compaction, the resultingpart is subjected to the present sinter-hardening process. This processcomprises heating the part to a certain temperature under an appropriateatmosphere and then cooling the part at a prescribed cooling rate.

The sinter hardening process may be conducted in a conventionalsintering furnace modified to allow the rapid cooling of the part thatis necessary, as detailed below. The part is preferably sintered at atemperature between 2040-2100° F., depending on the composition of thepart.

As previously noted, dimensional change and variation are known to berelated to graphite content. Graphite is the source of carbon in thepowder mixture that transforms iron to steel in the sintering furnace.Sintering must be performed in a controlled atmosphere to prevent theformation of oxides, avoid decarburization and remove oxides present onthe powder particles. A preferable atmosphere in the sintering furnacein which to conduct the sintering is mixed high purity nitrogen andhydrogen (elemental gases) to achieve minimal dimensional variation. Asuitable atmosphere may thus be, e.g., a 95%/5% blend of N₂/H₂.

Although less preferred, other atmospheres may include endothermic gas(generated by passing natural gas through a heated catalyst),disassociated ammonia (DA) (anhydrous ammonia disassociated by heating),and vacuum (absence of atmosphere, thereby absence of oxygen).Endothermic and DA inherently have more variation because the rawmaterials are relatively unpure. These unrefined product used to produceendothermic gas and DA contribute to excessive variation in percentageof oxygen and or carbon potential present in the sintering atmosphere,degrading sprocket dimensional characteristics. Additional variation canoccur due to the condition of the equipment necessary to process thenatural gas and anhydrous ammonia. The economics of vacuum sintering arepoor because of the need for a separate operation to remove pressinglubricants from the work pieces to avoid contamination of vacuum pumps.

By using a raw material system containing combinations of molybdenum,nickel, copper, manganese and carbon the proposed process heat treatsthe entire part during a modified sintering cycle, termedsinter-hardening. The modified cycle increases the cooling rate at thecritical temperature range of the cooling cycle. This avoids thereheating and rapid quenching necessary in conventional post-sinteringhardening practices, thereby dimensional change, distortion andvariation are minimized.

As mentioned above, the cooling of the part after sintering must takeplace at a rapid rate in order to affect a hardening of the metal part.A preferred rate is from 2.5-4.0° F./second. This rate of cooling can beaccomplished using, e.g., water-cooled jackets surrounding the parts asthey exit the high-heat region of the furnace or an efficient heatexchanger connected to the furnace. Such heat exchangers continuouslyexhaust the heated atmospheric gases in the oven, rapidly cool them, andcycle them back into the furnace. Other methods for rapid cooling may bepossible as well.

The invention has been described with reference to preferredembodiments. Modifications and alterations will occur to those ofordinary skill in the art to which the invention pertains, and it isintended that the following claims be construed literally and/oraccording to the doctrine of equivalents as encompassing all suchmodifications and alterations to the fullest possible extent.

1. An inverted tooth chain drive system comprising: a sprocketcomprising a plurality of teeth defined relative to respective toothcenters, said tooth centers spaced evenly in a circumferentialarrangement about an axis of rotation, each of said plurality of teethcomprising an engaging flank; an inverted tooth chain engaged with thesprocket and comprising a plurality of rows of links each structured forinside flank engagement with the sprocket, with leading inside flanks ofeach row of links projecting outwardly relative to the trailing outsideflanks of a preceding row of links, said leading inside flanks of eachrow positioned to make initial meshing contact with said engaging flankof one of said sprocket teeth; wherein at least some of said teeth arestandard teeth and other ones of said teeth are flank-relieved teeth,with said engaging flanks of said flank-relieved teeth negatively offsetrelative to their respective tooth centers as compared to said engagingflanks of said standard teeth relative to their respective toothcenters; and, wherein said plurality of teeth are separated from eachother by respective root surfaces, and said root surface adjacent andleading each flank-relieved tooth in terms of an intended direction ofrotation of said sprocket is a raised root surface located radiallyoutward with respect to said root surface adjacent each standard tooth.2. The inverted tooth chain drive system as set forth in claim 1,wherein: at an instant of initial meshing contact between one of saidrows of links of said inverted tooth chain and a first initial contactpoint on a standard tooth of said sprocket, a first initial contactangle is defined between a base reference line located at a 12 o'clockposition on said sprocket a first reference line that originates at saidaxis of rotation and that extends through said first initial contactpoint; at an instant of initial meshing contact between one of said rowsof links of said inverted tooth chain and a second initial contact pointon a flank-relieved tooth of said sprocket, a second initial contactangle is defined between said base reference line and a second referenceline that originates at said axis of rotation and that extends throughsaid second initial contact point; wherein said second initial contactangle is less than said first initial contact angle.
 3. (canceled) 4.The inverted tooth chain drive system as set forth in claim 1, wherein,when said inverted tooth chain fully meshes with a flank-relieved tooth,said inverted tooth chain contacts said raised root surface adjacentsaid flank-relieved tooth with which said inverted tooth chain is fullymeshed.
 5. The inverted tooth chain drive system as set forth in claim4, wherein each of said raised root surfaces is inclined inwardly towardsaid axis of rotation as said raised root surface extends forwardly awayfrom said engaging flank of said flank-relieved tooth toward a precedingtooth in terms of a direction of rotation for said sprocket.
 6. Theinverted tooth chain drive system as set forth in claim 1, wherein saidinverted tooth chain defines a chain pitch that is greater than atheoretical minimum chain pitch required for said chain to wrap saidsprocket.
 7. The inverted tooth chain drive system as set forth in claim1, wherein said flank-relieved teeth are defined with at least twodifferent flank-relieved tooth profiles, wherein said first and secondflank-relieved tooth profiles are defined with said engaging flanksthereof relieved a different amount as compared to each other relativeto their respective tooth centers. 8.-16. (canceled)
 17. A method ofmeshing an inverted tooth chain with a sprocket, said method comprising:rotating a sprocket while teeth of said sprocket are engaged with aninverted tooth chain so that leading inside flanks of each row of linksof said inverted tooth chain make initial contact with an engaging flankof a sprocket tooth and, after making initial contact, fully mesh withsaid sprocket, wherein said sprocket comprises: (i) a plurality ofstandard teeth having standard engaging flanks; and, (ii) a plurality offlank-relieved teeth having flank-relieved engaging flanks that arenegatively offset relative to said standard engaging flanks, said stepof rotating said sprocket comprising: rotating said sprocket a firstangular distance so that a first row of links of said chain fully mesheswith a first standard tooth; rotating said sprocket a second angulardistance so that a second row of links of said chain makes initialcontact with a second standard tooth that is preceded by said firststandard tooth, said second angular distance measured from an instantwhen the first row of links of said chain first becomes fully meshedwith said first standard tooth; and, rotating said sprocket a thirdangular distance so that a third row of links of said chain makesinitial contact with a flank-relieved tooth that is immediately precededby said second standard tooth, said third angular distance measured froman instant where said second row of links of said chain first becomesfully meshed with said second standard tooth of said sprocket, whereinsaid third angular distance exceeds said second angular distance.
 18. Aninverted tooth chain drive system comprising: a sprocket adapted forrotation about an axis of rotation and comprising a plurality of teetheach comprising an engaging flank, wherein at least some of said teethare standard teeth and other ones of said teeth are flank-relievedteeth, said engaging flanks of said flank-relieved teeth beingnegatively offset as compared to said engaging flanks of said standardteeth, said plurality of teeth separated from each other by respectiveroot surfaces, with the respective root surface adjacent and leading theengaging flank of each flank-relieved tooth in terms of an intendeddirection of rotation of said sprocket being a raised root surface thatis located radially outward as compared to the respective root surfaceadjacent and leading each standard tooth in terms of said direction ofrotation of said sprocket; an inside flank engagement inverted toothchain adapted for engagement with the sprocket and comprising aplurality of rows of links each structured for inside flank engagementwith the sprocket, with leading inside flanks of each row projectingoutwardly relative to the trailing outside flanks of a preceding row;wherein said sprocket and chain are structured such that: initialmeshing engagement between said chain and one of said standard teeth orone of said flank-relieved teeth includes initial contact between thetooth engaging flank and the leading inside flanks of a meshing chainrow of said chain; upon said inverted tooth chain fully meshing with oneof said standard teeth, said trailing outside flanks of said chaincontact the engaging flank of the standard tooth and toes of said chainare spaced from said root surface adjacent and leading said engagingflank of said standard tooth; and, upon said inverted tooth chain fullymeshing with one of said flank-relieved teeth, said trailing outsideflanks of said chain are spaced from the engaging flank of theflank-relieved tooth and the toes of said chain are in root contact withsaid raised root surface adjacent and leading said engaging flank ofsaid flank-relieved tooth.